In the manufacture of canned foodstuffs, e.g., meat products as ham, corned beef hash, chili and beef stew, the containers, usually metal cans, are filled with the foodstuff, covered with a metal end closure and sealed.
One of the disadvantages of canning meat products in metal containers is that the presence of the food product may cause the interior of the container to corrode, which corrosion results in contamination and deterioration of the food product.
Attempts to substitute certain inert synthetic resin materials, such as polyethylene and polypropylene for metal in the canning of foodstuffs, have encountered the disadvantage that such resin materials are excessively permeable to gases, such as oxygen, and the permeation of oxygen into the container causes an undesirable discoloration and a depreciation in the taste and qualities of the foodstuff.
The high gas permeability characteristics of synthetic resins, such as polyethylene, has resulted in containers fabricated from such resins being rejected in the packaging of oxygen sensitive comestibles where due to the chemical inertness of the resin, it might otherwise be employed to great advantage.
The art has devised a number of ways to reduce the gas permeability of polyethylene and other polyolefin resins. Included in these methods is the fabrication of the container from a thermoplastic resin wherein an inorganic mineral filler material such as clay or mica, or a gas impermeable resin such as Saran is incorporated in the resin. Examples of this art include U.S. Pat. Nos. 3,463,350, 3,923,190 and 4,102,974.
In U.S. Pat. No. 4,122,147 there is described a preferred method for the manufacture of containers from mineral filled polyolefin resins having improved gas barrier properties which are sealable with metal closures.
In the method disclosed in U.S. Pat. No. 4,122,147, the container is compression molded from a multilayer billet having a plurality of polyolefin layers, a first of these layers being comprised of a polyolefin resin composition exhibiting relatively high flexibility when molded, and a second of these layers being comprised of a polyolefin resin having incorporated therein a mineral filler which reduces the gas permeability of the resin. The first and second layers, when heated to a plasticized state and subjected to a compressive force, flow at non-uniform rates, the first layer flowing at a faster rate than the second layer.
In forming a container in accordance with U.S. Pat. No. 4,122,147, the billet is placed in a molding chamber and is compressed between a pair of die members with sufficient force to cause the layers of the billet to flow radially outward from between the die members at a differential rate into a mold cavity defining the sidewalls of the container. As the die members are advanced through the molding chamber, the faster flowing, more flexible, first layer is extruded into the mold cavity ahead of the remaining layers of the billet and forms the flange and exterior surface portions of the container whereas the slower moving second layers form the interior surface portions of the container.
The advancement of the die members through the molding chamber causes a continuous layer of multilayer material to be deposited and solidified within the mold cavity, the walls of which are maintained at a temperature below the solidification temperature of the extruded material. The extruded material, cooled to its solidification temperature, forms an integral solid hollow article having a multilayer side wall and bottom structure which is then ejected from the molding chamber.
Containers molded by the process of U.S. Pat. No. 4,122,147 from a polyethylene multilayer billet in which the second layer contains heavy loadings of mica, e.g., 40-50% by weight mica, when sealed with a metal end, have an oxygen leak rate when measured with a Mocon Oxtran 100 instrument of about 0.2 cc/100 in..sup.2 day atmos. at 73.degree. F.
Containers generally require an oxygen leak rate of less than 0.5 cc/100 in..sup.2 day atmos. at 73.degree. F. in order to be considered for the packaging of oxygen sensitive foodstuffs.
Although multilayer containers manufactured from mineral filled polyolefin resins have gas barrier properties that render them suitable for the packaging of oxygen sensitive foods, the containers have the disadvantage that when subjected to sterilization conditions, the gas barrier properties of the containers unexpectedly undergo a change to higher, more permeable values which then render the containers unsuitable for the packaging of the intended foodstuffs.
Thus in the packaging of foods such as fruit, beans, vegetables, meat and fish it is required that the foods be sterilized in the container. In such sterilization processes hermetically sealed containers having the food to be sterilized packaged therein are usually placed in a high pressure autoclave in which they are subjected to temperatures of 230.degree.-270.degree. F. with steam under pressure, e.g., 15 psig, for a sufficient time, e.g., 15-90 minutes, to destroy bacterial life.
When multilayer containers manufactured using mineral filled polyolefin resins such as mica filled high density polyethylene, are subjected to the sterilization conditions just described, the oxygen leak rate of such containers has been found to rise to above 0.5 cc/100 in..sup.2 day atoms. at 73.degree. F., which is unacceptable for the commercial use of such containers in the packaging of oxygen sensitive foods.